1. Field of the Invention
The present invention relates to medical devices and methods of making medical devices having bonded regions that employ a bonding agent with a secondary hydroxyl functional group. In addition, the present invention relates to intraluminal devices comprised of structural members having a plurality of locations wherein a bonding layer is present and may be comprised of one or more additives, particularly one or more radiopaque and/or lubricious additives. The present invention also relates to intraluminal devices comprised of contiguous heterogeneous structural elements further comprised of metallic members and non-metallic sections that are bonded together by a layer of phenoxy resin therebetween. The present invention also relates to the method of using phenoxy resin as a structural or coating layer.
2. Discussion of the Related Art
The prior art makes reference to the use of alloys such as Nitinol (Ni—Ti alloy), which have shape memory and/or superelastic characteristics, in medical devices that are designed to be inserted into a patient's body such as stents, anchors, fasteners, pins, spinal replacement parts and surgical tools. The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics, on the other hand, generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient's body, with such deformation causing a phase transformation. Once within the body lumen, the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original un-deformed shape by the transformation back to the original phase.
Alloys having shape memory/superelastic characteristics generally have at least two phases. These phases are a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenite phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensite phase.
Shape memory characteristics are imparted to the alloy by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e. a temperature above which the austenite phase is stable (the Af temperature). The shape of the metal during this heat treatment is the shape “remembered”. The heat-treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The metal in the martensite phase is then plastically deformed, e.g. to facilitate the entry thereof into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature causes the deformed martensite phase to transform to the austenite phase, and during this phase transformation the metal reverts back to its original shape if unrestrained. If restrained, the metal will remain martensitic until the restraint is removed. For medical device purposes, the Af temperature typically ranges from about 0 degrees Celsius to about 50 degrees Celsius, with a most preferable range being between about 10 degrees Celsius to about 37 degrees Celsius. However, for medical device applications where a purely martensitic behavior is desired, the Af temperature may be greater than about 37 degrees Celsius. Furthermore, for non-medical applications the Af temperature may be selected to be any temperature that optimizes the Nitinol stress—strain performance under the intended design conditions.
When stress is applied to a specimen of a metal such as Nitinol exhibiting superelastic characteristics at a temperature above which the austenite is stable (i.e. the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increases in stress are necessary to cause further deformation. The martensitic metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation.
If the load on the specimen is removed before any permanent deformation has occurred, the martensitic specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e. there is significant recovery in strain with only negligible corresponding stress reduction. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load, and to recover from the deformation upon the removal of the load, is commonly referred to as superelasticity or pseudoelasticity. Elastic tissues in the body have comparable stress-strain characteristics to superelastic Nitinol, thereby making Nitinol a desirable material for use in implantable medical devices.
Additionally, the surface of Nitinol and other appropriate materials may be passivated to form non-porous, inert oxides that offer good biocompatibility. The non-porous inertness of biocompatible surface oxides provide a technical challenge to device designs incorporating the attachment of two Nitinol parts together, or the attachment of Nitinol to other engineering materials such as plastics and other metals.
Conventional adhesives or glues generally do not provide optimal bonding when working with devices comprised of Nitinol or other material having inert passivated oxide surfaces. For the example of joining Nitinol surfaces, welding Nitinol to Nitinol may be a choice, but where the welding process requires other alloying or brazing metals, undesirable collateral changes in superelastic and shape memory properties may occur. Furthermore, the introduction of additional welding or brazing metals may result in a galvanic couple causing corrosion to occur. Moreover, the heat-affected zone adjacent to the weld area may negatively impact the desired superelastic and shape memory characteristics, including fatigue resistance. Surface oxides resulting from the high heat necessary for thermal welding may develop in a manner that compromise surface passivity, thereby causing the need for further material surface conditioning.
The use of friction welding may comparatively reduce the negative changes to the physical characteristics of Nitinol, however friction welding becomes increasingly difficult as the size of the bonded pieces decreases. Furthermore, friction welding is not a viable option for bonding Nitinol to dissimilar materials such as plastics. When working with dissimilar materials, which may include other metals or plastics, bonding options may yet become further limited. For example, the chemical resistant nature of thermoplastics exacerbates the difficulty of creating a good bond with the non-porous, inert oxides present on the surface of passivated Nitinol. Additionally, many adhesives and epoxies tend to be brittle, thereby further limiting the employment of Nitinol's advantageous superelastic and shape memory characteristics in medical device applications.